In general, as a material of a metal molding member such as an extrusion die, a material excellent in abrasion resistance such as forged die steel or cemented carbide is used. Since these materials are rigid, the workability is poor. Therefore, a processing method utilizing a discharge phenomenon as a processing principle, e.g., a wire electric-discharge machining or a carving electric-discharge machining in which workability is not generally harmed by strength of material, is widely used.
In the electric-discharge machining, a thermal melt elimination phenomenon of a material by discharge is used. Therefore, a molten residual layer (deformed layer) different from the base metal in quality will be formed on the surface of the machined object.
This layer has a structure transformed by melting and quenching, and includes micro cracks, a hardened layer (in cases where processing is executed in oil), a softened layer (in cases where processing is executed underwater) and/or tensile residual stress. Since high pressure will be applied to an extrusion die, if such a molten residual layer exists in the surface portion of the die, the micro cracks will act as starting points of destructions, resulting in a shortened die life. Furthermore, even in cases where a surface treatment is performed, the treatment may cause the exfoliation of the surface treated layer, which in turn shortens the life of the surface treatment.
As mentioned above, since the molten residual layer which exists in the surface of the processed portion of the metal molding member after the electric-discharge machining is harmful against the metal molding member, the molten residual layer is generally eliminated after the electric-discharge machining.
Conventionally, as a method for eliminating a molten residual layer of a metal molding member, various elimination methods, such as an Emery paper polish method, a gum polish method, a shot blast method, a barrel processing method and a honing method, are proposed (see, e.g., Japanese Unexamined Laid-open Patent Publication Nos. 10-156424, 11-123444, 11-244934 and 11-277131). Furthermore, a method for eliminating a molten residual layer by electrolytic polishing is also proposed (see, e.g., Japanese Unexamined Laid-open Patent Publication No. 9-41123).
However, the aforementioned method for eliminating a molten residual layer has the following drawbacks. That is, in cases where an electric-discharge machined portion has a minute configuration including, for example, a slot of less than 0.3 mm width, the method cannot cope with such a minute configuration. Furthermore, the strength of elimination processing varies from portion to portion and therefore a dull processed configuration will be generated, which deteriorates the processing dimensional accuracy.
Furthermore, the aforementioned electrolytic polishing elimination method has the following drawbacks. That is, in cases where the metal molding member is made of cemented carbide, some portions of a molding member will be eliminated preferentially by galvanic corrosion. Thus, the characteristics of the material cannot fully be used efficiently.
Furthermore, according to an elimination method using a common grinder, it was difficult to perform sufficient elimination processing in cases where a molding member has a minute processing configuration. Concretely, it was difficult to perform deep elimination processing (e.g., 10 times or more of width) when the slit has a width of about 0.3 mm.
Furthermore, there were another drawbacks as follows. When a popular disk-shaped grinder is used as a tool for the elimination processing, the elimination processing range will be limited. In cases where a tool having a columnar or rectangular cross-section other than a disk-shaped tool is used, if the diameter of the tool is less than 0.3 mm, it becomes difficult to attach the tool to the grinder straightly, causing a breakage of the tool because of the unbalanced load due to eccentricity, which results in unsatisfactory elimination processing.
It is an object of the present invention to provide a method for processing a metal molding member having a minute configuration capable of uniformly eliminating a molten residual layer (deformed layer) of the aforementioned minute configuration portion of the metal molding member subjected to electric-discharge processing with high accuracy whereby the life can be extended and the function can be enhanced.
It is another object of the present invention to provide a method for manufacturing a metal molding member, an extrusion die, a method for manufacturing an extruded member and an extruded member.
Other objects and advantages of the present invention will be apparent from the following preferred embodiments.